title: Structures of the flax-rust effector AvrM reveal insights into the molecular basis of plant-cell entry and effector-triggered immunity.
abstract: Fungal and oomycete pathogens cause some of the most devastating diseases in crop plants, and facilitate infection by delivering a large number of effector molecules into the plant cell. AvrM is a secreted effector protein from flax rust (Melampsora lini) that can internalize into plant cells in the absence of the pathogen, binds to phosphoinositides (PIPs), and is recognized directly by the resistance protein M in flax (Linum usitatissimum), resulting in effector-triggered immunity. We determined the crystal structures of two naturally occurring variants of AvrM, AvrM-A and avrM, and both reveal an L-shaped fold consisting of a tandem duplicated four-helix motif, which displays similarity to the WY domain core in oomycete effectors. In the crystals, both AvrM variants form a dimer with an unusual nonglobular shape. Our functional analysis of AvrM reveals that a hydrophobic surface patch conserved between both variants is required for internalization into plant cells, whereas the C-terminal coiled-coil domain mediates interaction with M. AvrM binding to PIPs is dependent on positive surface charges, and mutations that abrogate PIP binding have no significant effect on internalization, suggesting that AvrM binding to PIPs is not essential for transport of AvrM across the plant membrane. The structure of AvrM and the identification of functionally important surface regions advance our understanding of the molecular mechanisms underlying how effectors enter plant cells and how they are detected by the plant immune system.
authors.affiliation: School of Chemistry and Molecular Biosciences, Australian Infectious Diseases Research Centre and Institute for Molecular Bioscience, University of Queensland, Brisbane, QLD 4072, Australia.
authors.forename: Thomas
authors.initials: T
authors.lastname: Ve
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Williams
authors.forename: Ann-Maree
authors.initials: AM
authors.lastname: Catanzariti
authors.forename: Maryam
authors.initials: M
authors.lastname: Rafiqi
authors.forename: Motiur
authors.initials: M
authors.lastname: Rahman
authors.forename: Jeffrey G
authors.initials: JG
authors.lastname: Ellis
authors.forename: Adrienne R
authors.initials: AR
authors.lastname: Hardham
authors.forename: David A
authors.initials: DA
authors.lastname: Jones
authors.forename: Peter A
authors.initials: PA
authors.lastname: Anderson
authors.forename: Peter N
authors.initials: PN
authors.lastname: Dodds
authors.forename: Bostjan
authors.initials: B
authors.lastname: Kobe
mesh_terms: D000595:Amino Acid Sequence; D001487:Basidiomycota / Q000235:genetics / Q000276:immunology* / Q000502:physiology; D018360:Crystallography, X-Ray; D019597:Flax / Q000166:cytology / Q000276:immunology* / Q000382:microbiology; D005656:Fungal Proteins / Q000737:chemistry / Q000276:immunology* / Q000378:metabolism; D054884:Host-Pathogen Interactions / Q000276:immunology; D015151:Immunoblotting; D018613:Microscopy, Confocal; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D010716:Phosphatidylinositols / Q000276:immunology / Q000378:metabolism; D059828:Plant Cells / Q000276:immunology / Q000382:microbiology; D010935:Plant Diseases / Q000235:genetics / Q000276:immunology* / Q000382:microbiology; D018515:Plant Leaves / Q000235:genetics / Q000378:metabolism; D030821:Plants, Genetically Modified; D011485:Protein Binding / Q000276:immunology; D055503:Protein Multimerization; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D014026:Nicotiana / Q000235:genetics / Q000378:metabolism
chemical_list: D005656:Fungal Proteins; D010716:Phosphatidylinositols
keywords: avirulence protein; innate immunity; lipid binding; plant cell internalization; plant disease resistance